Treatment of molten metal using arc heat and vacuum

ABSTRACT

Hydrogen and oxygen are removed from molten steel by simultaneously subjecting molten steel to agitation, an alternating current heating arc, and two successive levels of vacuum, one level of vacuum being relatively low and above the glow range and of relatively long duration to substantially decrease the oxygen content of the molten steel; the other level of vacuum being relatively high and below the glow range and of relatively short duration to substantially decrease the hydrogen content of the molten steel.

Finkl 1 51 Tan. 1%,, 11972 154] TREATMENT E MOILTEN METAL 1,555,313 9/1925 Rohn ..75/49 USING ARC HEAT AND VACUUM 1,555,314 /1925 Rohn ..75/49 [72] Inventor: Charles W. Finkl, Chicago, 111. FOREIGN PATENTS OR APPLICATIONS [73] Assignee: A. lFinlrl 8:. Sons Company, Chicago, 111. 1,078,540 8/1967 United Kingdom ..75/49 [22] Filed: May 211, 1968 OTHER PUBLICATIONS [21] Appl' NO; 739,586 Degas, Refine, Reheat in One Ladle," The iron Age, pp.

48- 49 (Dec. 23, 1965). [52] US. Cl 75/11, 75/10, 75/49 4 51 1111. C1. C2lc /52, C22d 7/00, (:2 1 c 7 Primary Examl7er-Wmswn Douglas 58 1 Field of Search ..75/10, 49, 1 1, 12; 148/l6.5 AMI-Wm EmmmerPet =r Rosenberg Attorney-Parker, Caner & Markey [56] References Cited [57] ABSTRACT UNITED STATES PATENTS Hydrogen and oxygen are removed from molten steel by 3,024,102 3/1962 Brown ..75/l0 simultaneously subjecting molten steel to agitation, an alter- 32031781 8965 g nating current heating arc, and two successive levels of 3,279,912 10/1966 Death ....75/l0 vacuum, one level of vacuum being relatively low and above 2 1 1 H1965 T y the glow range and of relatively long duration to substantially 3,236,635 2966 Flnkl decrease the oxygen content of the molten steel; the other 3,501,289 3/1970 F1nk1 ...75/49 level f vacuum being relatively high and below the 1 1 11641 5/1970 -F 75/49 range and of relatively short duration to substantially decrease 31512957 5/1970 Bmtzmann the hydrogen content of the molten steel. 2,997,760 8/1961 Hanks 75/10 3,072,982 l/1963 Gordon "75/10 C Claims, N0 Drawings TREATMENT OF MOLTEN METAL USING AlltC llllllEfit'll AND VACUUM This invention relates generally to the treatment of molten metal and specifically to a method and system for adding heat to molten metal. Although the invention is adaptable to and useful in many applications, for convenience of description it will be described as applied to the postmelting treatment of molten steel, and specifically to the vacuum-degassing treatment thereof.

in recent years higher and higher quality steel has been de manded for many applications, such as the bearing industry. To meet the high requirements several vacuum degassing processes have been developed and put into use, each of which functions to a greater or lesser extent to lower the quantity of deleterious gases and/or undesirable inclusions, such as oxide inclusions, in the final product. The problem of heat loss is, however, common to all of these vacuum degassing processes at this time. This invention eliminates the temperature loss in an economical, efi'rcient, and safe manner.

Accordingly a primary object of the invention is to eliminate the heat loss problem now common to all molten metal treatment procedures, such as vacuum degassing.

Another object is to enable chemical treatment of molten steel to be carried out at the most desirable time in the treatment period. For example, the addition of aluminum and silicon to meet specifications may be made with the use of the invention at a time which will not cause inhibition of the deleterious gas removal action, such as the well-known carbon monoxide reaction which lowers the content. it is now known that active deoxidizers should be added late in a vacuum treatment cycle but with the present time temperature limitations on vacuum treatment even the late addition may be at a time when the steel is not in the optimum chemical condition for the admission of the deoxidizers.

Another object is to increase melt shop efficiency by reducing melting unit time for each heat and thereby increase the throughput of existing equipment.

Yet another object is to lower the oxygen content of molten steel by a vacuum treatment at a vacuum level which, at this time, is considered relatively low and inefficient. Removal of oxygen at this relatively low vacuum has a number of advantages, including the ability to degas with a lower capacity, and hence less expensive, vacuum system than is now considered feasible.

Other objects and advantages of the invention will be apparent from the following detailed description.

In its most basic form, the invention comprises the subjec tion of agitated molten steel to the heating effect of an AC- heating are under a vacuum which is outside of the arc glow range but of a magnitude sufficient to remove deleterious gases, particularly oxygen, from the steel.

in an even more effective form, the invention includes, in addition to the above concept, the subjection of the agitated metal to a vacuum below the level of incipient glow for the purpose, among others, of lowering the quantity of hard to remove gases, such as hydrogen, this treatment being carried out at relatively high vacuum.

In this specification, the term high vacuum is used to denote a vacuum of a level sufficient to effectively remove all of the deleterious gases, including hydrogen. Specifically, with the equipment to be hereinafter described, the vacuum may be on the order of about 2 or 3 mm. absolute or below, this degree of vacuum being required, with the hereinafter described vacuum system, to lower the hydrogen content below the upper tolerable hydrogen content limit for flakefree steel. The term relatively low" vacuum will be used in this specification to denote a vacuum sufficient to remove substantial quantities of oxygen. it is impossible to assign a specific vacuum level since the degree of vacuum depends upon the equipment used, the type of metal being treated and the time available. In a conventional vacuum-degassing chamber, the vacuum level may usually lie in the range from 30 to several hundred mm. of mercury, or above, although, for practical operational reasons, the vacuum level may usually lie in the range of -200 mm. of mercury. lt may even be possible to operate at substantially atmospheric pressure under a controlled atmosphere in a closed chamber.

The sequence in which the aforementioned steps is carried out is not critical to the attainment of the objects of the inven' tion. it is, however, contemplated at this time that the invention can be most efficiently and economically employed if subj ection to a vacuum above the glow range is carried out before subjection to the vacuum below the level of incipient glow since this procedure utilizes the pumpdown time period.

it is likewise not essential that the subjection of the agitated metal to the higher vacuum when a multistep process is employed be done in the absence of a heating are. it might, for example, be desirable to maintain an ACheating arc of relatively low-heat-input capacity (as contrasted to the high-heatinput capacity of the predegassing arc) during the finish degassing step. if for example existing conditions are such that the glow effect is not too serious at relatively low power input levels, the low energy level AC-arc may be maintained during the finish-degassing" step to help control the rate of heat loss. it should be understood that the finislrdegassing period is more characterized by its high-vacuum level than its absence of a heating are.

For convenience of description, the terms preheating" and predegas will be used to denote that portion of a cycle in which a high energy input capacity AC arc is maintained during a relatively low vacuum. The terms "post-heat" and finish-degas will be used to denote that portion of a treatment cycle in which the metal is subjected to a high vacuum usually, through not always, in the absence of a heating are. An arc of reduced voltage may for example be used, the reduced voltage being too low to create the glow discharge phenomena. This terminology will be employed irrespective of the sequence in which the physical steps are performed.

One great advantage of the invention is that the steel maker is not now limited to any particular treatment procedure; he may indeed add or omit steps as desired. For example, although the usual procedure may be to predegas, then finish degas, it is quite feasible to retum to the predegassing phase before ending the process. This can be extremely important in situations in which the temperature of "the steel is not at the desired level at the conclusion of the first two steps. Thus, if the temperature is too low the steel may merely be subjected to the low vacuum-high energy level heating are and brought back to proper temperature. Furthermore, substantial quantities of alloying materials may be added at the most desirable point in the cycle. And further, the invention makes possible the holding of a batch of steel for what is considered an extremely long period of time. This may be useful for example in a systems in which the quantity of molten metal required for a casting process is greater than the capacity of metal produced by the melting unit. By holding molten metal until a second batch of metal is produced by the melting unit, a melting system of relatively low capacity.

Irrespective of whether a low or high energy input are is employed, the metal must be agitated. Agitation stirs in alloys, brings remote undegassed or virgin portions of the bath to the surface where it is exposed to the gas-removal action of the vacuum, creates uniformity of temperature throughout the bath, and protects the equipment from overheating. Gas purging is the preferred mode of agitation since it is instantly and fully responsive to control, can be selectively regulated to optimize the efficiency of the electrical performance of the system, provides a carrier medium into which deleterious gases may migrate (and thereby leave the steel) at locations far beneath the surface of the steel, and is the simplest to operate, safest, and most reliable and inexpensive form of agitation. Gas purging has the ability to effectively raise the level of the bath beneath an electrode so that the arc is buried in the froth of the boil. That is, the arc may be positioned within the molten metal, but not in direct contact with metal in its liquid phase.

Apparatus suitable for practice of the invention may include a vacuum tank into which a ladle of molten metal to be treated is placed. Altemately, of course, the ladle or other moltenmetal-holding receptacle may form a portion of the vacuum enclosure. The vacuum enclosure may include a single or multipart cover assembly and a plurality, usually three, of graphite or other nonconsumable-type electrodes carried in the uppermost section. One or a plurality of purging plugs are carried in the shell of the ladle near the lower portion thereof, each plug being located beneath one of the electrodes. A suitable ar rangement is disclosed in US. Pat. No. 3,236,635. The rate of gas admission to each purging location may be separately controlled as by a valve in the supply line. Each plug is connected to a source of purging gas usually located outside the vacuum enclosure. lf chemical reactions, in addition to mechanical agitation, are desired between the gas and the steel, suitable gases may be employed of which CC], is an example. Preferably, however, the gaseous purging agent will be chemically inactive with respect to the metal undergoing treatment. in the treatment of carbon and low alloy steels, He, Ar, N C C0 and dry air may be employed. It should also be understood that the purging agent need not be in a gaseous condition at the instant of its contact with the molten metal. The

agent might, for agent might, for example, be in solid form and be melted and then vaporized by contact with the hot metal.

An important function of the purging agent during are on periods is to create a circulation within the bath which will tend to equalize the temperature. If no agitation were employed in a bath of substantial depth the heat would be concentrated at the surface and eventually some portion of the system would fail from overheating, such as a stopper rod or the receptacle wall.

The purging gas can also serve the important function of controlling the arc length at the electrodes. Experience has shown that even when the height of the electrodes above the molten metal is uniform from electrode to bath a voltage variance between phases may exist in the absence of a purging agent. By regulating the rate of gas admission at one or more of the gas emission points the level of the boil under vacuum conditions can be controlled to increase or decrease individual arc lengths. By controlling the effective level of the bath under each electrode, the heat input and power consumption may be balanced from electrode to electrode which achieves optimum electrical efficiency. If for any reason the electrical characteristics should change during a treatment cycle, the purging rate beneath the electrodes can be instantly varied to reestablish electrical balance. Altemately, of course, each electrode may be provided with its individual electrode control, but this is very much more costly.

Ladle additions of charge materials including alloys and/or slag-forming materials may be made at any point in the cycle by hoppers or other addition means associated with the vacuum tank assembly. Any alloy may be added, though it may be most efficient to add those alloys which are best adapted for addition subsequent to treatment in the melting unit, such as those which do not readily form oxides. Addition of highly deoxidizing alloys such as Al and Si may advantageously be made late in the cycle to minimize inhibition of O removal by the carbon monoxide reaction in the melt.

A suitable apparatus for practicing the invention is illustrated in US. Pat. Nos. 3,50l,289 and 3,501,290, said disclosure being incorporated herein by reference. If desired, the electrodes may be blunt-nosed and the refractory sleeves omitted in addition to relocating the purging plugs as above described.

In its most basic form the invention comprises the subjection of molten steel to the simultaneous action of a relatively low vacuum, an AC-heating arc, and a purging agent. Table 1 gives the results of heats treated in this fashion.

From the above table it will be noted that the oxygen content was very substantially decreased (nearly 50 percent in heat 91) even though the lowest vacuum reached was in the low vacuum range (only 155 mm. Hg in heat 91 Offurthersignificance is the fact that the temperature drop was minimal (only 10 F. in heat 91) after 25 minutes of vacuum treatment. Past experience indicates that molten metal loses 6 to 7 F. per minute in the vacuum system in which this heat was treated. Therefore, a very much greater temperature loss to l75 F. in the case of heat 91 for example) would be expected in the absence of the heating arc. This result is even more significant when it is remembered that the equipment, including the vacuum tank, the refractory radiation shield, and even the ladle, to some extent, acts as a heat sink which takes heat from the molten steel until a rough equilibrium condition exists. During treatment, the amount of He purgant was regulated to ensure roughly balanced electrical characteristics from electrode to electrode.

The advantage of the invention can be appreciated when it is understood that for FX-steel the aim-teem temperature is 2,8302840 F., and that in the absence of the arc the before vacuum" temperature (and therefore the tap temperature) for a 25-minute or longer vacuum treatment would have had to be much higher (2,8903,0l5 F. in heat 91). The higher before vacuum" and tap temperature gives rise to the numerous disadvantages which are set out in greater detail in copending application Ser. No. 575,897 and elsewhere in the art.

A multistep variation of the invention is disclosed in the following table. in each instance the metal was initially" predegassed at a relatively low vacuum using a high energy input AC-arc, and thereafter finish degassed under a high vacuum in the absence of the arc, the metal being agitated by a gas purgant at all times.

It will be noted that in all heats for which pin-tube samples were obtained the final hydrogen value was within the flake limit and the oxygen content was substantially lowered-from 75 percent in heat 103 to 26 percent in heat 102. A comparison of the actual before and after temperature drops experienced with what could be expected for similar nonarced vacuum periods will show in each instance that the temperature of the steel was clearly controlled by the use of the arc.

The type of purging gas is not especially critical to the successful practice of the invention. Table 3 shows the results of using two or more purging gases for each heat.

The final temperature values and oxygen results show no significant difference over the corresponding values in Tables 1 and 2.

1 The relative noncriticality from a chemical standpoint of the sequence in which the predegassing and finished gassing steps are performed can also be seen from the above table. In heat 98, the finish degas step was performed first (high vacuum without are) followed by predegassing (relatively low vacuum with arc). From an economic standpoint, however, it may be more desirable to first predegas and then finish degas since the rate of pumpdown for the finish degassing period, when performed initially, may be relatively slow due to the heavy gas load on the ejectors and accordingly time during which the arc could be utilized is wasted.

The vacuum level at which the predegassing or preheating step is run is determined by the vacuum level at which glow discharge begins. The glow phenomena has been well described in the literature of which the discussion on pages 71-73 of High Vacuum Engineering, Barrington, Prentice- Hall, 1963, is an example. in the practice of this invention it is an undesirable condition since the glow discharge, or ball of tire, is inefiicient for heating the molten metal. To the contrary, the glow phenomena serves mainly to heat up the vacuum chamber and exposed mechanism therein which can lead to rapid equipment failure.

in an air system, the vacuum level at which glow discharge would occur at different voltages is indicated by the curve in the following graph which is based on the aforesaid High Vacuum Engineering reference.

This curve indicates that with a decrease in pressure, starting from 760 mm. Hg, glow discharge will occur at a progressively lower voltage until a vacuum in the region of 500 to 800 microns is reached. As the pressure is further decreased from the 500-800 micron range, the voltage at which glow discharge occurs progressively increases with decreasing pressure. This indicates that after passing through the area above 3 s I .r

5 Age/M d re/r/om/ s. r/zwraz/ y s g a" if E m R g z g M /a' /0 M m Torr-m7 the curve, that is, into the region of very high vacuum of about 500 microns Hg or less, a high energy are may again be struck and maintained without establishing glow discharge.

Actual experience proves that for voltages in glow discharge begins to occur in the 30 mm. to 90 mm. range, with the most common inception occurring in the 8090 mm. Hg range. The exact reason for the inception of glow discharge at this level is not known with certainty at this time. It is thought that since a pure air atmosphere does not exist in the vacuum chamber during degassing the right-hand portion of the curve is depressed to some extent. That is, it is thought that additional charged particles, including charged metal ions, are present which support glow discharge at a higher vacuum for a given pressure than would be indicated by the above-mentioned graph.

At the vacuum levels required to produce flake free steel, which experience indicates is in the range of 2-3 mm. Hg or below, oxygen as well as hydrogen will be effectively removed. As 'earlier mentioned, oxygen is effectively removed at relatively low vacuums. For convenience of reference a vertical line has been shown to indicate the approximate beginning of the effective H removal vacuum range. It should be understood, however, that the line is approximate only.

Although experience indicates that the establishment of the glow phenomena is primarily a function of vacuum and voltage level, the type of purging gas appears to have some effect.

Referring to Table 3, it was noted that nitrogen and dry air begin to glow at around 90 mm. of mercury while CO did not begin to glow until a vacuum level of about 80 mm. mercury was reached. Helium glows at around 85 mm. of mercury. In other heats tabulated in this specification glow was not definitely established until levels of 50 and 30 mm. of mercury were reached. It thus appears that glow begins in the range of about 30 to 90 mm. of mercury with inception occurring most frequently in the 8090 mm. of mercury range. For this reason and, to some extent, the limitations of the vacuum system, the arc-on periods were run at a range above 100 mm. of mercury to avoid the glow phenomena.

In table 4 the results of a heat in which an arc of approximately 1.9 MW was maintained during the finish-degas period are tabulated. During the period in which the lower voltage arc was on, no glow discharge occurred. With equipment of sufiiciently large-current-carrying capacity, it appears glow discharge at any vacuum level can be avoided by operating at a sufficiently low voltage. 5

No temperature readings were taken at the crossover point between predegas and finish degas since the vacuum was not broken. in all probability, however, the temperature of the bath was somewhat higher than 2,850 F. at the crossover point and the effect of the low voltage are was to slow the rate of heat loss during the finish degassing period.

If desired the metal may be rearced at the conclusion of a conventional predegas-finish-degas sequence. This may be particularly desirable when, for some reason, such as a heat tapped too cold, the temperature after the finish-degas period is too low for proper teeming. Table 5 gives the results of several such heats.

In heat 88, the temperature reading between the second and third arc-on periods was 2,790 F. The final 25 minutes of arcon therefore raised the temperature of the heat by 80.

would be performed in a situation where further processing,

such as a continuous casting process, requires a greater charge of metal than the melting unit can produce in one heating cycle.

ln heat a delay occurred subsequent to taking the afterarc temperature and the heat cooled to 2,740 F. which is too cold for proper teeming and, in the absence of any means for increasing the heat, would require return to the furnace for further heat. At this point the heat was returned to the treatment chamber and in 10 minutes at mm. Hg the temperature was raised 30. Since the tank was still hot there was little heat sink which indicates that once the system is heated to a point at which the rate of heat loss to the surrounding atmosphere has stabilized to a fairly steady rate, the process is capable of increasing the temperature of the metal as desired.

in heat 94 temperature readings taken at selected intervals indicated that twice the heat dropped to 2,810 F.

From table 6 it is apparent that postrnelting treatment periods of up to an hour and more are quite feasible with the existing equipment. Longer periods can, of course, be obtained with larger and heavier equipment.

Table 6 also illustrates the ability to make substantial postmelting charge additions without cooling the heat too greatly.

In heat 95, over 1,800 pounds of FeMn and Fe-Cr were added during the cycle. The calculated heat loss for this quantity of charge material in a heat of the size was approximately 45 F. Despite this temperature loss and the loss attributable to heat sink, the steel was still near the permissible pouring temperature at the conclusion of treatment. In heat 88 over 500 pounds of sand and lime were added after 40 minutes of vacuum.

All heats in the foregoing tables were run at about 17,000 amps, with the range being from about l6,000 to 19,000.

From the foregoing description of the invention it is apparent that modifications may be made by those having ordinary skill in the art. Accordingly, the scope of the invention should be defined, not by the terms of the foregoing description, but solely by the hereinafter appended claims when construed in light of the pertinent prior art.

I claim:

I. In a method of removing hydrogen and oxygen under vacuum from molten steel, the steps of providing molten steel from a melting unit,

subjecting the molten steel to an altemating-current heating arc which is formed between an alternating current heating arc source and the molten steel, circulating portions of said molten steel from locations which are remote from the surface of the molten steel to the surface during maintenance of said are, and

simultaneously with the subjection of the steel to the heating arc and the circulatory effect derived from the aforesaid circulation of the steel, I

subjecting the molten steel to a vacuum at two successive vacuum stages,

one vacuum stage being a relatively low vacuum which is maintained for a sufficiently long period of time to substantially decrease the oxygen content of said molten steel,

said vacuum stage being of magnitude above the glow range as established by the vacuum, the composition of gases in the system, and the electrical characteristics of the heating arc,

the other vacuum stage being a relatively high vacuum which is maintained for a sufficiently long period of time to effectively substantially decrease the hydrogen content of said molten steel,

said last mentioned vacuum stage being of a magnitude below the upper end of the glow range as established by molten steel is subjected to the vacuum stage above the glow range before it is subjected to the vacuum stage below the upper end of the glow range.

3. In a method of removing hydrogen and oxygen under vacuum from molten steel, the steps of providing molten steel from a melting unit,

subjecting the molten steel to a vacuum at two successive vacuum stages,

one vacuum stage being a relatively low vacuum which is maintained for a sufficiently long period of time to substantially decrease the oxygen content of said molten steel,

the other vacuum stage being a relatively high vacuum which is maintained for a sufficiently long period of time to effectively substantially decrease the hydrogen content of said molten steel,

the duration of said last-mentioned vacuum stage being a shorter period of time than the first mentioned vacuum stage,

circulating portions of said molten steel from locations which are remote from the surface of the molten steel to the surface during maintenance of vacuum, and,

simultaneously with the subjection of ,the steel to the aforesaid one vacuum stage and the circulatory effect derived from the aforesaid circulation of the steel,

subjecting the molten steel to an altemating-current heating are which is fonned between an altemating-current heat ing-arc source and the molten steel,

said one vacuum stage being of a magnitude above the glow range as established by the vacuum, the composition of gases in the system, and the electrical characteristics of the heating arc,

said other vacuum stage being of a magnitude below the upper end of the aforesaid glow range.

4. The method of claim 1 further characterized in that the voltage of the heating are during said other vacuum stage is sufficiently low to substantially eliminate glow discharge at the second vacuum stage. 5. The method of claim 1 further characterized in that the vacuum is above about 30 mm. Hg. absolute during the time the vacuum is outside the glow range. 6. The method of claim 1 further characterized in that the vacuum is above 100 mm. Hg. absolute during the time the vacuum is outside the glow range. 7. The method of claim 1 further characterized in that the vacuum of said one stage is above about 30 mm. Hg. ab-

solute, and the vacuum of said other stage is about 2 mm. Hg. absolute or below during at least a substantial portion of the time said other stage vacuum is maintained.

8. The method of claim 1 further characterized in that the steel is circulated by the upward passage of a purging as. 9. lhe method of claim 8 further characterized in that the purging gas is selected from the group consisting of the inert gases, dry air, CO and CO 10. The method of claim 8 further characterized in that the molten steel is subjected to the vacuum stage above the glow range before it is subjected to the vacuum stage below the upper end of the glow range.

11. In a method of removing hydrogen and oxygen under vacuum from molten steel, the steps of providing molten steel from a melting unit,

subjecting the molten steel to a vacuum at two successive vacuum stages,

one vacuum stage being a relatively low vacuum which is maintained for a sufficiently long period of time to substantially decrease the oxygen content of said molten steel,

the other vacuum stage being a relatively high vacuum which is maintained for a sufiiciently long period of time to effectively substantially decrease the hydrogen content of said molten steel,

the duration of said last mentioned vacuum stage being a shorter period of time than the first mentioned vacuum stage,

circulating portions of said molten steel from locations which are remote from the surface of the molten steel to the surface during maintenance of vacuum by the upward passage of a purging gas, and,

simultaneously with the subjection of the steel to the aforesaid one vacuum stage and the circulatory effect derived from the aforesaid purging gas,

subjecting the molten steel to an alternating current heating are which is formed between an alternating current heating arc source and the molten steel,

said one vacuum stage being of a magnitude above the glow range as established by the vacuum, the composition of gases in the system, and the electrical characteristics of the heating arc,

said other vacuum stage being of a magnitude below the upper end of the aforesaid glow range.

12. The method of claim 8 further characterized in that the vacuum is above about 30 mm. Hg. absolute during the time the vacuum is above the glow range,

13. The method of claim 8 further characterized in that the vacuum of said one stage is above about 30 rrim. Hg. ab-

solute, and

the vacuum of said other stage is about 2 mm. Hg. absolute or below during at least a substantial portion of the time said other vacuum stage is maintained.

14. The method of claim 1 further characterized in that firstly, the altemating-current arc-heating source comprises a plurality of nonconsumable electrodes arranged to are directly to the molten steel, and

secondly, a purging gas emission area is located beneath one or more electrodes, and further including the step of balancing the electrical power load from electrode to electrode by controlling the boil beneath each electrode by controlling the rate of emission of purging gas beneath one or more electrodes.

15. The method of claim 14 further characterized in that the vacuum is above about mm. Hg absolute during the time the vacuum is outside the glow range. 

2. The method of claim 1 further characterized in that the molten steel is subjected to the vacuum stage above the glow range before it is subjected to the vacuum stage below the upper end of the glow range.
 3. In a method of removing hydrogen and oxygen under vacuum from molten steel, the steps of providing molten steel from a melting unit, subjecting the molten steel to a vacuum at two successive vacuum stages, one vacuum stage being a relatively low vacuum which is maintained for a sufficiently long period of time to substantially decrease the oxygen content of said molten steel, the other vacuum stage being a relatively high vacuum which is maintained for a sufficiently long period of time to effectively substantially decrease the hydrogen content of said molten steel, the duration of said last-mentioned vacuum stage being a shorter period of time than the first mentioned vacuum stage, circulating portions of said mOlten steel from locations which are remote from the surface of the molten steel to the surface during maintenance of vacuum, and, simultaneously with the subjection of the steel to the aforesaid one vacuum stage and the circulatory effect derived from the aforesaid circulation of the steel, subjecting the molten steel to an alternating-current heating arc which is formed between an alternating-current heating-arc source and the molten steel, said one vacuum stage being of a magnitude above the glow range as established by the vacuum, the composition of gases in the system, and the electrical characteristics of the heating arc, said other vacuum stage being of a magnitude below the upper end of the aforesaid glow range.
 4. The method of claim 1 further characterized in that the voltage of the heating arc during said other vacuum stage is sufficiently low to substantially eliminate glow discharge at the second vacuum stage.
 5. The method of claim 1 further characterized in that the vacuum is above about 30 mm. Hg. absolute during the time the vacuum is outside the glow range.
 6. The method of claim 1 further characterized in that the vacuum is above 100 mm. Hg. absolute during the time the vacuum is outside the glow range.
 7. The method of claim 1 further characterized in that the vacuum of said one stage is above about 30 mm. Hg. absolute, and the vacuum of said other stage is about 2 mm. Hg. absolute or below during at least a substantial portion of the time said other stage vacuum is maintained.
 8. The method of claim 1 further characterized in that the steel is circulated by the upward passage of a purging gas.
 9. The method of claim 8 further characterized in that the purging gas is selected from the group consisting of the inert gases, dry air, CO and CO2.
 10. The method of claim 8 further characterized in that the molten steel is subjected to the vacuum stage above the glow range before it is subjected to the vacuum stage below the upper end of the glow range.
 11. In a method of removing hydrogen and oxygen under vacuum from molten steel, the steps of providing molten steel from a melting unit, subjecting the molten steel to a vacuum at two successive vacuum stages, one vacuum stage being a relatively low vacuum which is maintained for a sufficiently long period of time to substantially decrease the oxygen content of said molten steel, the other vacuum stage being a relatively high vacuum which is maintained for a sufficiently long period of time to effectively substantially decrease the hydrogen content of said molten steel, the duration of said last mentioned vacuum stage being a shorter period of time than the first mentioned vacuum stage, circulating portions of said molten steel from locations which are remote from the surface of the molten steel to the surface during maintenance of vacuum by the upward passage of a purging gas, and, simultaneously with the subjection of the steel to the aforesaid one vacuum stage and the circulatory effect derived from the aforesaid purging gas, subjecting the molten steel to an alternating current heating arc which is formed between an alternating current heating arc source and the molten steel, said one vacuum stage being of a magnitude above the glow range as established by the vacuum, the composition of gases in the system, and the electrical characteristics of the heating arc, said other vacuum stage being of a magnitude below the upper end of the aforesaid glow range.
 12. The method of claim 8 further characterized in that the vacuum is above about 30 mm. Hg. absolute during the time the vacuum is above the glow range.
 13. The method of claim 8 further characterized in that the vacuum of said one stage is above about 30 mm. Hg. absolute, and the vacuum of said other stage is about 2 mm. Hg. absolute or below during at least a substantial portion of the time said other vacuum stage is maintained.
 14. The method of claim 1 further characterized in that firstly, the alternating-current arc-heating source comprises a plurality of nonconsumable electrodes arranged to arc directly to the molten steel, and secondly, a purging gas emission area is located beneath one or more electrodes, and further including the step of balancing the electrical power load from electrode to electrode by controlling the boil beneath each electrode by controlling the rate of emission of purging gas beneath one or more electrodes.
 15. The method of claim 14 further characterized in that the vacuum is above about 100 mm. Hg absolute during the time the vacuum is outside the glow range. 